A US study involving overweight men and women suggests that exercise might actually help us shed weight – undercutting a widespread theory that exercise, by itself, is worthless for weight loss.

But the findings also indicate that, to benefit, we may need to exercise quite a bit.

In theory, exercise should contribute substantially to weight loss. It burns calories. If we do not replace them, our bodies should achieve negative energy balance, use stored fat for fuel and shed pounds.

But life and our metabolisms are not predictable or fair, as multiple exercise studies involving people and animals show. In these experiments, participants lose less weight than would be expected, given the energy they expend during exercise.

The studies generally have concluded that exercisers compensate for the energy they expend during exercise, either by eating more or moving less. These compensations were often unwitting but effective.

Some researchers had begun to wonder, though, if the amount of exercise might matter. Many of the past human experiments had involved about half an hour a day or so of moderate exercise – the amount generally recommended by current guidelines to improve health.

But what if people exercised more, some researchers asked. Would they still compensate for all the calories that they burned?

To find out, scientists from the University of North Dakota and other institutions decided to invite 31 overweight, sedentary men and women to a lab for measurements of their resting metabolic rate and body composition.

Scientists wondered if more exercise would overcome dieters’ compensatory calories. Photo: Getty

The volunteers also recounted in detail what they had eaten the previous day and agreed to wear an activity tracker for a week.

One random group began walking briskly, or otherwise exercising, five times a week until they had burned 300 calories, which took most of them about half an hour.

During the week, these volunteers burned 1500 extra calories with their exercise program.

The other group began working out for twice as long, burning 600 calories a session, or about 3000 a week.

The exercise program lasted for 12 weeks. The researchers asked their volunteers not to change their diets or lifestyles during this time and to wear the activity monitors for a few days.

After four months, everyone returned to the lab and repeated the original tests.

The results must have been disconcerting for some of them. Those men and women who had burned about 1500 calories a week with exercise turned out to have lost little if any body fat. Some were heavier.

But most of those who had walked twice as much were thinner. Twelve of them had shed at least 5 per cent of their body fat during the study.

The researchers then used mathematical calculations, based on each person’s fat loss (if any), to determine whether and by how much they had compensated for their exercise.

The men and women in the group who had burned 1500 calories a week with exercise proved to have compensated for nearly 950 of those calories, the numbers indicated.

Those in the other group had also compensated for some of the calories they had burned, and to almost the exact same extent as those who had exercised less. They had added back about 1000 calories a week.

But since they had expended 3000 calories a week, they had wound up with a deficit of about 2000 calories from exercise and lost fat, the researchers concluded. The findings were published in the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology.

Those who had burned more calories had lost more weight – even though they had also compensated. Photo: Getty

How the volunteers had compensated was not absolutely clear, said Kyle Flack, an assistant professor at the University of Kentucky, who conducted the experiment as part of his graduate research.

People’s resting metabolic rates had not changed during the study, he said, whichever group they had been in. Their activity monitors also showed few differences in how much or little they moved during the day.

So the caloric compensation must have involved overeating, he said.

But the volunteers did not think so.

“Their food recall did not show differences” in how much they reported eating at the start and end of the study, Dr Flack said.

“I think they just did not realise that they were eating more,” he said.

There are probably also complicated interconnections between exercise, appetite and people’s relationships to food that were not picked up during the study and could affect eating and weight, he said.

But the results from the experiment were encouraging, if cautionary.

“It looks like you can lose weight with exercise,” Dr Flack said.

But success might require more exertion of our bodies and will than we might hope, he said.

“Thirty minutes of exercise was not enough” in this study to overcome the natural drive to replace the calories burned during a workout.

“Sixty minutes of exercise was better,” he said.

But even then, people replaced about a third of the calories they expended during exercise.

“You still have to count calories and weigh portions” if you hoped to use exercise to control your weight, he said.

–New York Times